The available bit is difficult to answer. You never know who is available until you ask
Realistically we should be able to buy from more or less anyone below us in the Premier League if we put enough money into it. Similarly, there are only a relatively small number of clubs internationally who are big enough to fend us off from a financial perspective.
Those are probably Real Madrid, Barcelona, Atletico Madrid, Juventus, PSG, Bayern Munich and Borussia Dortmund. Now there may be players at the likes of both Milan clubs, Roma, Monaco, Lyon, Porto, Sevilla, etc etc who don't fancy a move to us as they are looking for CL clubs, but they shouldn't really be out of our grasp financially.
To go back to the point about who is better...
In this country, keepers from the teams below us who I'm confident are better than Joel: Butland, Schmiechel, Forster. That's about it. Pickford is a potentially good keeper, but I'm not on the Heaton fan bus.
Outside of England, I guess we all start relying on the limited foreign footy that we watch but Leno and Rui Patricio instantly spring to mind as top keepers who aren't at massive clubs, whilst the likes of Donarumma who looks to be one of the continent's next top stars are probably unrealistic.